1. Field of the Invention
The present invention relates to a worm biasing structure.
2. Description of Related Art
Typically, in a motor-driven power steering apparatus, a rotating force of an auxiliary motor for the generation of an assistance force is transmitted to a pinion shaft and the like via a speed reduction mechanism. For example, the speed reduction mechanism is formed by a worm gear mechanism.
For example, when the motor-driven power steering apparatus is a pinion assistance steering apparatus, the speed reduction mechanism includes a worm (drive gear) connected to an output shaft of the motor, and a worm wheel (driven gear) that meshes with the worm and is fixed to a pinion shaft. In the speed reduction mechanism disclosed in JP-A-2013-208932, a compression coil spring biases the worm toward the worm wheel in a biasing direction (preload direction), and exerts a preload on the worm wheel so as to obtain good meshing of the worm and the worm wheel even though the worm and/or the worm wheel may be worn out, and prevent backlash between the worm and the worm wheel.
Specifically, a resin-made bearing holder accommodates a radial bearing that rotatably supports the worm, and is accommodated in a housing hole of a housing. The bearing holder is split at a location in a circumferential direction of the bearing holder, and has a C shape when seen from an axial direction thereof. A pair of guide surfaces is provided at an inner circumferential surface of the bearing holder so that the radial bearing is interposed, in the direction perpendicular to the biasing direction, between the pair of guide surfaces when the radial bearing is accommodated in the bearing holder. The guide surfaces extend in the biasing direction, and guide the bearing in the biasing direction. That is, the pair of guide surfaces guide the bearing while interposing the bearing therebetween in a perpendicular direction perpendicular to the biasing direction. The C-shaped bearing holder is accommodated in the housing with the diameter of the bearing holder being slightly reduced.